This invention relates to an amplifying circuit, especially an amplifying circuit suitable for implementation in NMOS technology.
It is relatively simple to design amplifying circuits having high gain where a complementary transistor technology is available. There are, however, circumstances where it is desirable to use only one conductivity type of transistor, for example, in circumstances where it is technologically difficult to form complementary transistors. As indicated in a paper entitled "A High Performance All-Enhancement NMOS Operation Amplifier" by I A Young published in the IEEE Journal of Solid-State Circuits, Vol SC-14, No 6, December 1979 at pages 1070 to 1077 and a paper entitled "An Integrated NMOS Operational Amplifier with Internal Compensation" by Y P Tsividis and P R Gray published in the IEEE Journal of Solid-State Circuits, Vol SC-11, No 6 December 1976 at pages 748 to 753, when only enhancement mode NMOS transistors of one conductivity type, generally n conductivity type, are available, the gain obtained for each amplifier stage is to a first order dependent on the square root of the ratio between the conduction channel width/length (W/L) ratio of the input NMOS transistor to that of the load NMOS transistor. Accordingly, to achieve high gain in an amplifying stage a large area input or driver transistor is required which inevitably results in a large parasitic capacitance. The overall gain of the amplifying circuit may be increased by using a number of amplifier gain stages as described in the aforementioned papers, but this inevitably increases the area required by the amplifying circuit and, moreover, makes the use of frequency compensation capacitors necessary.
The use of a form of boot-strapping, as described by, for example, B J Hosticka in a paper entitled "Improvement of the Gain of MOS Amplifiers" Published in the IEEE Journal of Solid Circuits, Vol SC-14, No 6, December 1979, pages 1111-1114 may be helpful but it may still be necessary to provide many gain stages to provide a reasonable open loop or small signal gain.